The production of copolymers from N-vinyllactams and hydrophobic comonomers by free-radical polymerization is known. The production of such copolymers takes place in an organic solvent, for example an alcohol. Usually, the polymerization is carried out with reflux of the solvent. The hydrophobic monomers which are more readily volatile than the N-vinyllactams pass, in this way, into the gas phase and into the condensate.
For many applications there is a desire for copolymers which form clear solutions in water, i.e. the FNU value of a 5% strength by weight solution should be <10, preferably less than 5. However, there is the problem that the varying reactivities and the varying polarity of the monomers can lead to increases in the concentration of the hydrophobic monomers, which has the result that non-water-soluble homopolymers can be formed from the hydrophobic monomers. Even in small amounts, depending on the comonomer in the range from 20 to 1000 ppm, such homopolymers lead to turbidity of an aqueous solution of the copolymers. The increases in the concentration of hydrophobic monomers can arise in particular in the gas phase and in the condensate, and on the reactor wall and the surface of the polymerization medium.
U.S. Pat. No. 5,395,904 describes the polymerization of vinylpyrrolidone and vinyl acetate through controlled polymerization by the feed method. An alcoholic solvent is used which can comprise up to 50% by weight of water.
U.S. Pat. No. 5,319,041 describes the preparation of copolymers of vinylpyrrolidone and vinyl acetate through polymerization by the feed method with control of the polymerization temperature.
U.S. Pat. No. 5,502,136 describes a method of producing copolymers of vinylpyrrolidone and vinyl acetate by the feed method, where the feeds are controlled via a scheme defined by certain mathematical formulae.
U.S. Pat. No. 4,520,179 and U.S. Pat. No. 4,554,311 describe the polymerization of vinylpyrrolidone and vinyl acetate with t-butyl peroxypivalate as initiator in water or water/alcohol mixtures. The initiator used therein permits the production of copolymers with a narrow molecular weight distribution, but it does not lead to water-soluble products with an FNU value <10.
EP-A 161 describes a method of producing copolymers of vinylpyrrolidone and vinyl acetate, where, after the polymerization, an afterpolymerization with certain initiators is carried out. However, the polymers have high residual contents of vinyl acetate and are not sufficiently free from turbidity.
EP-A 795 567 describes the production of copolymers of vinyllactams and hydrophobic monomers by polymerization in aqueous solution.
EP-A 418721 discloses the production of copolymers of vinylpyrrolidone and vinyl esters which dissolve in water to give clear solutions, in which, at a certain point in the polymerization, solvent exchange is undertaken in order to remove volatile constituents. This method is relatively complex.
DE-OS 22 18 935 describes the copolymerization of N-vinylpyrrolidone with various water-soluble and water-insoluble comonomers. Use is made here of water-insoluble initiators which are used in the form of a finely divided suspension in an aqueous solution of the copolymers. In the case of the water-insoluble comonomers, however, this likewise does not lead to the desired water-soluble copolymers with an FNU value <10.